This invention relates to a spectrophotometer and more particularly to an ultraviolet and visible light absorbance detector of a kind provided with a flow cell having an elongated sample cell opening and a reference opening parallel thereto. Such a spectrophotometer is frequently used in high-power liquid chromatography.
When components of a sample contained in a carrier liquid are to be analyzed by measuring their ultraviolet and visible light absorbance, it has been known to use a detector of the kind described above. The optical system of such a detector of the so-called variable wavelength type may be formed so as to direct a beam of light from a source such as a D.sub.2 lamp or tungsten lamp to a grating by means of mirrors and through a slit with a rectangular opening and to form an image of the slit with the light dispersed by the grating on an area of the flow cell covering the light-receiving windows of both the sample cell opening and the reference opening.
FIGS. 1A, 1B and 1C show an example of such a prior art flow cell 1, having a Z-shaped zigzagging carrier liquid flow route 3 therethrough. A portion of this Z-shaped carrier liquid flow route 3, indicated by numeral 5, serving as what was referred to as "the sample cell opening" above, is formed so as to be parallel to the optical path of the dispersed light made incident onto the flow cell 1. The length of the light path through the sample cell opening 5 may be about 10 mm, its inner diameter being about 1 mm. A portion of the dispersed light guided to the flow cell 1 passes through a converging lens 6, enters the sample cell opening 5 through its light-receiving window 7 on the incident side, traverses the sample cell opening 5, passes through another converging lens 8 and is projected out through a light-emitting window 9. As shown in FIG. 1C, the flow cell 1 is also provided with a reference opening 11 with inner diameter 1 mm, extending parallel to the sample cell opening 5. Another portion of the dispersed light guided to the flow cell 1 is introduced into the reference opening 11 through its light-receiving window 13 and is emitted out through its light-emitting window 15.
The image of the slit formed on the flow cell 1 by the optical system is elongated in the direction perpendicular to the direction of the spectral dispersion (as indicated by numeral 17 in FIG. 1A). The sample cell opening 5 and the reference opening 11 are arranged such that the image 17 covers both the light-receiving windows 7 and 13 of both the sample cell opening 5 and the reference opening 11.
The portions of light which pass through the sample cell opening 5 and the reference opening 11 are each detected by a separate detector such as a photodiode. The intensity thus detected is used to calculate absorbance ("absorbance conversion") from which target components in the sample are detected.
With a detector of the kind described above, it is very important to improve its S/N ratio. One method of doing so is to increase the signal intensity, but the length and the inner diameter of the sample cell opening cannot be increased indiscriminately because the detected separate chromatographic peaks should not be overly broadened and the sensitivity of the detector should be kept high. In other words, it is not feasible to increase the signal intensity.
The S/N ratio can be increased also by reducing the noise (N). In order to reduce the noise, it may be suggested that the throughput of light inside the sample cell should be increased, but the inner diameter of the sample cell opening is not to be increased beyond about 1 mm because the monochromatic characteristic of the incident light should be maintained. In other words, its throughput is uniquely determined and cannot be hoped to be increased. In summary, it is not an easy task to reduce the absorption noise caused by the sample cell opening.